It's probably the last reason that first caused the start of this curious habit. Around the 17th century the element was believed to have magical powers. As it could (reputedly) cure all ills which were regarded as having demonic origin, the Church kept such cures under it's own control. Self medication was treated therefore, as a sin. When the church's powers began to wane around 1700, after a couple of plagues and the 30 Years War; some farmers and peasants began to eat arsenic, and grooms fed it to their horses.
It seems the Styrian arsenic eaters took their arsenic as an oxide; arsenic trioxide, which probably slowed the absortion rate considerably. This trioxide form has a white porcelain appearance, allowing users to gauge purity, and give themselves a correct dose. It seems users would eat up to several hundred milligrams per day, usually with bread and bacon. Reputedly people would consume such doses over up to 40 years or more with out apparent harm.

Considering that the current standard for arsenic levels in drinking water is often 50 parts per billion or less, these stories seem incredible. But in 1860 a British doctor, H.E. Roscoe proved that these peasants did indeed consume arsenic, and not zincoxide which has a similar appearance. How they managed to take such high doses however is not known, although there are present day populations that are also exposed to high levels of arsenic in their environment, which do not exhibit symptoms of poisoning. In the town of San Pedro de Atacama in Chile, the local water supply has for the past several thousand years contained arsenic levels of 600 microgrammes per litre, and the residents appear to be fine!

One of the elements, a solid substance resembling a metal in its physical properties, but in its chemical relations ranking with the nonmetals. It is of a steel-gray color and brilliant luster, though usually dull from tarnish. It is very brittle, and sublimes at 356° Fahrenheit. It is sometimes found native, but usually combined with silver, cobalt, nickel, iron, antimony, or sulphur. Orpiment and realgar are two of its sulphur compounds, the first of which is the true arsenticum of the ancients. The element and its compounds are active poisons. Specific gravity from 5.7 to 5.9. Atomic weight. Symbol As.